Issue

Wafer Dicing and Thinning

04/01/2007

The Long, Slow March Toward Transition

BY JEFFREY ALBELO, Electro Scientific Industries, Inc.

If you had told me seven years ago that the industry would consider spending around $30 million for lithography tools (immersion 193 nm), I would have laughed out loud. That is even truer of tools ranging $50 million (extreme ultraviolet [EUV]), and yet here I sit, looking at the numbers, continually amazed by how much, how far, and how long this industry will spend, go, and wait to extend the frontiers of transistor technology. My vexation comes on the heels of Intel’s recent revelation of their whole 45-nm solution using metal gates and, of all things, hafnium. Pardon the pun, gentle reader, for I am herald to the transition next to darken our doorways.

Although abrasive mechanical sawing - dicing saw - has been around for nearly as long as silicon dioxide gate dielectric, it too will be replaced. Make no mistake, it will be tough to displace this workhorse from the outer limits of the BEOL, but move on it must. If design-rule trend lines are to be believed, the time on which this mechanical dicing technology transacts is equal parts; borrowed and running. The equation for understanding this is both simple and entirely first-order: cost drives change. When the cost of goods sold (COGS) become too high, technological innovations are embraced and put into production as quickly as possible. Until the requisite level of discomfort is reached, however, the incumbent technology persists. With the advent of thinner dice, higher chip density per wafer, low-k and ultra-low-k dielectrics, and previously unfathomably low defect limits, the rock-saw approach is no longer viable.

Benefits of thinner device substrates are well-known, and often lamented by the back-end designers, though it is not typically for the assumed thermal benefit as much as it is for the volumetric dictates of stacked-die architectures, which are becoming increasingly common. Chief among drivers for vertically integrated devices is the rapid and quasi-exponential expansion in I/O bandwidth requirements. Like all the improvements we have squeezed from FEOL unit operations, the benefits have not been without their attendant costs. In this case, thinning the Si substrate has ushered in a nearly exponential increase in die failures as a function of wafer thickness after backgrinding. In addition to these saw-driven failures (a.k.a. rolling the dice, or at least popping them off the dicing tape), mechanically induced stress (cracking: micro and/or macro) in the substrates has contributed to an explosion in post-package device failures, which present themselves during thermal-aging studies. There are also changes driven by applications-specific requirements, such as the robust growth in smart card devices, set to replace paper passports within the next 5 years. Changes such as these are driving aggressive substrate thickness roadmaps (devices on 25-µm Si), which further sours an already stressed relationship. Mechanical dicing and thinner wafers - strange bedfellows indeed.

Low-k integration is a promise wrapped in its own set of problems, which appear to have been tamed to the extent that the ITRS low-k roadmap has recently undergone only minor changes. 2005 was the first time in 10 years it wasn’t pushed out. Mechanical dicing technology is struggling with these materials. Delamination, passivation peeling, and micro-crack propagation have all contributed to risky products presently in production all over the world. If nothing else, this industry is risk-averse, which is motivation enough to find a solution.

Presently, these problems are addressed by slowing the saw process, but this is not sustainable. This is Moore’s universe. New technology must supplement then supplant the old. These new technologies bring increased cost and complexity. Dice before grind (DBG), laser scribing, and laser dicing round out the present bestiary of understudy technologies competing with, or in some cases complementing, each other and existing mechanical dicing techniques. Sooner rather than later, an alternative to mechanical dicing must be conjured and adopted. It most certainly must involve a non-contact process, which promises improved yields and long-term reliability. Several scenarios are likely. All are dependant on the results of early experiments presently underway all over the world. Primarily they are aimed at closing the gaps between state-of-the -art and the ideal solution. The mere prospect of 0.5-2.0% line-yield improvement, with a concomitant throughput increase over the incumbent approach, is more than enough to get tongues wagging among industry insiders and early adopters alike.

I know it’s common in this industry to prognosticate the untimely demise of a technology. If one doubts this, I offer by way of example the long and oft-forecast death of optical lithography, which was alive and well as of a few hours ago. No matter, the winds of change are beginning to blow.